1. Field of the Invention
The present invention relates to a semiconductor optical device and more particularly, to a semiconductor optical device having an optical waveguide which represents nonlinear refractive index change due to control light absorption.
2. Description of the Prior Art
To realize high-speed optical-fiber communication and optical information processing systems, higher speed operations are essential for signal light controlling devices used therefor.
With conventional semiconductor optical devices used for these applications, signal light has been controlled by electric signals applied to the corresponding optical devices, which is termed the "electrical control" of optical devices. Recently, a technique termed the "optical control" of optical devices" has been attracting a great deal of attention, in which signal light is controlled by control light instead of an electric signal.
An example of such the conventional semiconductor optical devices utilizing the above optical control is disclosed in the Japanese Non-Examined Patent Publication No. 6-186602 (July, 1994), the application No. of which is 4-341863.
This conventional optical device has an optical waveguide that represents any nonlinear change of refractive index due to absorption of control light introduced thereinto. The control light is introduced into the waveguide and is absorbed therein, exciting carriers, i.e., electrons and holes, in the waveguide. The carriers thus excited causes the "band-filling" effect, so that a nonlinear change of the refractive index of the waveguide is produced. The refractive index change of the waveguide phase-modulates the signal light propagating through the waveguide.
In the case of the nonlinear change of the refractive index due to the band-filling effect, such the refractive index change appears in an extremely short time (e.g., 1 ps or less) after the generation or excitation of the carriers. This means that the beginning time of the device operation is very short. The refractive index change, however, disappears in a comparatively long time that varies dependent upon the lifetimes (typically, 1 ns or less) of the excited carriers. This means that the ending time of the device operation is considerably long, which prevents a desired high-speed operation of the device.
With the above conventional device, since the electrostatic field is applied to the waveguide, the excited carriers in the waveguide are quickly swept or extracted from the waveguide, which means prompt recovery of the refractive index change or reduction of the long ending time. Thus, the operation speed of the optical device can be hastened.
The conventional semiconductor optical device disclosed in the Japanese Non-Examined Patent Publication No. 6-186602 has a structure shown in FIG. 1. As seen from FIG. 1, this device has a ridge R5 extending along the center line at the top of the device.
In FIG. 1, an n-Al.sub.x Ga.sub.1-x As (x=0.07) lower cladding layer 53 with a thickness of 2 .mu.m is formed on a surface of a semi-insulating GaAs substrate 52. The substrate 52 is doped with Si at a concentration of 10.sup.18 atoms/cm.sup.3. The lower cladding layer 53 also is doped with Si at a concentration of 10.sup.18 atoms/cm.sup.3.
An undoped GaAs core layer 54 with a thickness of 0.5 .mu.m is formed on the lower cladding layer 53. An undoped Al.sub.x Ga.sub.1-x As (x=0.07) upper cladding layer 55 with a thickness of 0.2 .mu.m is formed on the core layer 54.
A p-Al.sub.x Ga.sub.1-x As (x=0.07) upper cladding layer 56 with a thickness of 0.6 .mu.m is formed on the upper cladding layer 55. The layer 56 is doped with Be at a concentration of 10.sup.18 atoms/cm.sup.3.
A p-GaAs cap layer 57 with a thickness of 0.2 .mu.m is formed on the upper cladding layer 56. The layer 57 is doped with Be at a concentration of 10.sup.18 atoms/cm.sup.3.
The upper cladding layer 56 and the cap layer 57 are selectively etched at each side of the ridge R5. The surface area of the upper cladding layer 55 also is etched at each side of the ridge R5.
A SiO.sub.2 film 57 is formed to cover the exposed top faces of the upper cladding layer 55 and the exposed side faces of the upper cladding layers 55 and 56 and the cap layer 57.
An upper electrode 59 is formed on the SiO.sub.2 film 57. The upper electrode 59 is in Ohmic contact with the exposed top face of the GaAs cap layer 57. A lower electrode 51 is formed on a back surface of the GaAs substrate 52. The lower electrode 51 also is in Ohmic contact with the substrate 52.
A backward bias voltage V (e.g., V=10 V) is applied across the upper and lower electrodes 59 and 51, so that an electrostatic field is applied to the undoped GaAs core layer 54.
An optical waveguide is formed in the core layer 54 right below the ridge R5 to extend along the ridge R5. Signal light is introduced into the waveguide together with control light along an arrow shown in FIG. 1. The control light thus introduced is absorbed in the waveguide by the core layer 54 to generate carriers, i.e., electrons and holes, therein, producing any nonlinear change of refractive index of the waveguide.
The carriers thus generated are promptly swept or extracted from the waveguide by the electrostatic field produced by the bias voltage V. In detail, the electrons are swept from the waveguide toward the lower cladding layer 53 and the holes toward the upper cladding layer 55. As a result, desired quick disappearance of the nonlinear refractive index change is realized.
The signal light introduced into the waveguide is phase-modulated by the above refractive index change in response to the introduction or absorption of the control light. Since the nonlinear change of refractive index is quickly generated due to the excited carriers and the sweep of the excited carriers performed by the electrostatic field quickly terminates the change, the signal light is controlled or phase-modulated at a higher speed than the case of no electrostatic field.
Typically, nonlinear refractive index change of a semiconductor material due to excited carriers increases dependent upon the density of the excited carriers. The increase rate of the refractive index change, however, decreases as the density of the carriers increases. In other words, the nonlinear refractive index change tends to saturate as the density of the carriers becomes large.
Therefore, if the excited carriers are distributed uniformly in the waveguide, a necessary density of the excited carriers for obtaining a desired refractive index change or phase shift is smaller than the case that the carriers are not distributed uniformly. In other words, the distribution uniformity of the excited carriers in the waveguide reduces the necessary energy of the control light for operating a semiconductor optical device.
With the conventional semiconductor optical device of FIG. 1, the bandgap wavelength of the waveguide is constant over the entire waveguide, which is equal to .lambda..sub.g0, and the density of the excited carriers, which is d.sub.0 at the entrance of the waveguide, decreases drastically from the entrance to the exit along the waveguide, as shown in FIG. 2. This means that the uniformity of the carrier density is very small.
Therefore, although the carrier density is relatively large in the vicinity of the entrance, an obtainable refractive index change does not become large in proportion with its large carrier density due to the saturation of the nonlinear optical effect. As a result, the control light needs to have a higher energy to realize a satisfactory, nonlinear change of refractive index over the full length of the waveguide.